Method of electrolytic coating



Aug. 11,1953 J. 5. BUSER METHOD OF ELECTROLYTIC COATING Original FiledJuly 5, 1944 I N VEN TOR.

X m Y mm 5 D N mm m an w 8 U A B0 f mr5 5 "am mo Y B Patented Aug. 11,1953 UNITED STATES PATENT OFFICE METHOD OF ELECTROLYTIC COATING JosephS. Buser, deceased,

late of Marland Continuation of application Serial No. 543,318, July 3,1944. This application March 10, 1949,

Serial No. 80,724

5 Claims.

The present invention relates to the electrolytic coatin of a basemetal, and more particularly to the treating of the strip immediatelyafter it has been coated With a suitable metal coating and to therecovery of the electrolyte carried from the plating cells during thecoating operation. The invention is particularly applicable to theelectrolytic manufacture of tin plate and will be described herein asapplied to the manufacture of such a product. However, it Will beapparent, as the description of the invention proceeds, that it is notlimited to the manufacture of such a product but may be utilized inconnection with the coating of various types of base metals with varioustypes of coating metals.

This application is a continuation of application Serial No. 543,318,filed July 3, 1944, by Joseph S. Buser, deceased. Application Serial No.543,318 is now abandoned.

In the manufacture of electro-coated tin plate, the strip-like metal ispreliminarily treated and is then coated by passing it through anelectrolytic bath containing a plurality of anode blocks of tin. Duringits passage through the electrolyte, the strip is the cathode and thetin from the anode blocks is plated on the strip as it passes throughthe electrolyte. In one such system the strip is successively passedthrough a plurality of plating cells where the strip is plated on oneside and is then passed successively through another group of platingcells where it is coated on the other side. In another system which isin use commercially, the strip is passed vertically downwardly andupwardly through a suitable bath past the anode blocks and is coated onboth sides at the same time. The present invention is applicable toeither of these systems as it relates to the treating and handling ofthe strip after it has been coated.

As the strip leaves the final plating cell, it carries with it anappreciable amount of the electrolyte. The electrolyte employed in anyof these coating or plating systems is quite expensive and,consequently, it is highly desirable to recover as much of it as ispossible and return it to the plating line. However, in the recovery ofthe electrolyte and the return of it to the plating line several factorsof importance must be taken into consideration. In the first place, itis necessary that the rinsing solution be of such character that it willnot have a deleterious effect upon the product. And in the second place,the rinsing solution, after it has Washed the electrolyte from thecoated strip, must be of such character that it can be returned to theplating in the manufacture of tin plate.

system without appreciably modifying the electrolyte from the desiredcomposition. The present invention provides a method and apparatuswhereby a high proportion of the electrolyte carried from the platingcells by the strip can be recovered and returned to the plating lineWithout adversely afiecting the product or the electrolyte in thesystem. In fact, as will be pointed out hereinafter, the method whichthis invention provides is decidely advantageous from the standpoint ofthe quality of the finished product.

Difiiculty has heretofore been encountered in obtaining a product havinga bright finish thereon. As a consequence, various proposals have beenmade for eliminating the dull finish of the electro-coated product. Ithas been proposed to burnish the surface by mechanical brushingapparatus. It has also been proposed to subject the coated strip totemperatures sufiiciently high to cause the coating thereon to flow andthereby eliminate the dull mat finish. Flow brighteners of severaldifferent types have been proposed, including (1) the passing of thestrip through a heated oil bath and (2) the heating of the stripelectrically to the temperature required for refiowing the coating. Ithas been found, however, that the film of electrolyte carried from theplating bath adversely affects the appearance of the resulting producteven though it is subjected to a flow brightening operation. It has beenfound that if the film-of electrolyte is not eliminated from the stripit becomes hydrolyzed to stannous or stannic hydroxide and that duringthe passage of the strip through the flow brightener it is converted tothe oxide and that this results in a chalky appearance in the finalproduct. Accordingly, by the present invention one not only recovers theelectrolyte which is carried from the plating cells but also providesfor the P elimination of a substantial proportion of the solution whichmay later be hydrolyzed to stannic or stannous hydroxide and adverselyaffect the finished product.

Various types of electrolytes may be utilized For example, tin chloride,tin fluoride, and tin sulphate baths have been utilized heretofore. Theelectrolyte may contain other salts of the acid constituent employed,such as'sodium chloride, sodium fluoride, potassium chloride, potassiumfluoride or potassium sulphate. Where a tin chloride or tin fluoridebath is employed, the solution carried from the plating cell includesstannous chloride or fluoride or stannic chloride or fluoride, or boththe stannous and stannic chlorides or fluorides. Where a tin sulphatebath is employed, the solution carried from the plating cells includesstannous sulphate or stannic sulphate, or both.

Where the solution carried from the platin cells includes stannouschloride or stannic chloride, or both, in the absence of propertreatment the stannic chloride will hydrolyze to stannic hydroxide andthe stannous chloride will hydrolyze to stannous hydroxide. Similarhydrolysis takes place where tin .sulphate is present in theelectrolyte; that is, the stannous and stannic sulphates hydrolyze tostannous and stannic hydroxides. The presence of these hydroxides isobjectionable in view of the effect on the resulting product.Consequently, it has been found that in recovering the solution which iscarried from the plating cells it is desirable to prevent the formationof the objectionable stannous hydroxide or stannic hydroxide on thestrip to any substantial extent. Where untreated water is used in therecovery process, an excess of these objectionable compounds is formedand must be removed from the strip before it passes to the refiowingapparatus. It has been found, however, that by utilizing a propersolution for rinsing the strip a substantial proportion of the solutioncarried from the plating cells can be recovered without the formation ofan excessive amount of stannous or stannic hydroxides.

In the operation of a line for the electrocoating of strip, the bath isusually maintained at an elevated temperature, for example, in theneighborhood of 135 F. Considerable evaporation takes place during theoperation of the line and, consequently, provision must be made forreplacing the evaporated solution. It has been found that this can beaccomplished y supplying to the rinsing apparatus an amount of waterapproximately equal to the evaporation losses and collecting the rinseWater and returning it to the plating cells.

In the accompanying drawing there is shown, for purpose of illustrationonly, apparatus which may be used in carrying out the present invention.However, the invention is not limited to the use of any specificapparatus as it may be otherwise practiced or embodied without departingfrom the scope thereof. As shown in the drawing, which is diagrammaticonly, the strip S passes through the final plating cell 2 and betweenthe contact rolls 3. It is t carried past sprays 4 which spray on thestrip a rinsing solution from the recovery tank 5, the nature of whichwill be described hereinafter. The strip is then passed between squeezerollers 6. It is then again sprayed by sprays l with the rinsingsolution from the recovery tank. It then passes between squeeze rollers8. It again passes between sprays 9 which spray solution from therecovery tank on it, and it is then passed between squeeze rollers l!)adjacent the recovery tank. During the passage of the strip between thealternately arranged sprays and squeeze rollers, it is located above acollecting tank II in which the solution sprayed onto the strip by thesprays and squeezed from the strip by the squeeze rolls is collected.The alternate spraying of the rinsing solution and squeezing thesolution from the strip results in the elimination of a substantialproportion of the solution carried from the plating cells. Theconcentration of tin chloride in the electrolyte i approximately 4ounces per gallon. The first set of sprays l dilutes this toapproximately 1 ounce per gallon. The succeeding sprays and squeezerolls further dilute the solution carried by the strip from the platingcells, so that at the time the strip enters the recovery tank theconcentration of tin chloride is down to approximately .3 ounce pergallon.

After alternately passing between the sp ays and squeeze rolls, thestrip is passed into and through the recovery tank 5, which contains therinse solution, where it is further rinsed. The strip then passes fromthe tank, between squeeze rolls [2, and then over a tank 13 where it issprayed with cold water by means of sprays M. The strip then passeseither to the hot rinse described and claimed in the copendingapplication Serial No. 546,091 or to drying apparatus and then to therefiowing apparatus, in the event that the coated strip is to besubjected to a refiowing operation.

The rinsing solution contained in the recovery tank 5 and which issprayed onto the strip by the sprays l, 1 and 9 is water which has beentreated by the addition of a salt or an acid which will lower the pHvalue of the water to a point where objectionable hydrolysis of the tinsalt carried from the plating cell is eliminated. Where an acid is addedto the rinse water, the pH value of the rinsing solution should bemaintained at approximately 1.90 or under. However, where salts areemployed, it has been found that the pH value may be somewhat higher,for example, approximately 2.5, without substantial hydrolysis takingplace and without adversely affecting the electrolyte solution when therinsing solution is returned to the system.

Where the solution carried from the p at cells includes stannouschloride or stannic Chloride, it has been found that excellent resultscan be obtained by utilizing a rinse water which has been treated withsodium fluoride, sodium bifluoride or hydrofluoric acid. Hydrochloricacid also may be used, but it a definite tendency to etch the tincoating, and hence it has been found preferable to utilize hydrofluoricacid instead. Various other salts of hydrofluoric and hydrochloric acidsmay be used so long as the pH value of the rinse water is lowered to asuitable extent and so long as they will not have a deleterious effectupon the final product.

Where the solution carried from the plating cells contains stannous orstannic sulphates, sulphuric acid, phenol disulphonic acid and varioussalts of these acids may be used to bring the pH value of the rinseWater down to the required value.

Where the electrolyte contains tin chloride, the pH of the bath isordinarily maintained between approximately 2.5 and 3.5. Therefore, theuse of large quantities of hydrofluoric acid in the rinse water woulddeleteriously affect the pH of the electroplating bath since the rinsewater is returned to the electroplating bath. For this reason, it ispreferable to use a salt such as sodium fluoride or sodium bifluoride inthe rinse water instead of the acid since a higher pH value in the rinsewater can be maintained without hydrolysis taking place where the saltinstead of the acid is used.

The water for the rinsing spray may be fed to the recovery tank throughan inlet l5. The treating agent may be added to the water before itenters the recovery tank or it may be added directly to the tank. Freshwater is supplied to the recovery tank at the rate necessary to replacethe rinsing solution fed to the sprays. Since the rinsing solution isreturned to the plating system in the manner described below, the rateat which fresh water is supplied to the recovery tank is maintained atthe amount required to replace the loss due to evaporation of theplating bath and strip carry-out. The rinsing solution is fed to thesprays i, 5 and 9 from the outlet it. The rinsing solution which issprayed onto the strip and a high percentage of the solution. carriedfrom the plating cells are collected in the collecting tank E], which islocated below the sprays and the squeeze rolls. Phis collecting tank IIhas an outlet I! which is connected by suitable piping to one of thecells in the plating line. Instead of being connected directly to theplating cells, the outlet B! may be connected to the supply tank for thecells. The rinsing solution returned to the plating system is fed to thesystem at a sufiicient rate to take care of the loss .of the platingsolution occasioned by evaporation and by the strip passingtherethrough.

As stated above, after the strip passes through the recovery tank, it issprayed with cold water by the sprays Hi. This rinse water is not fedinto the electrolytic circulatory system but is discharged to the sewerthrough an outlet 58.

The final rinsing of the strip with cold water hydrolyzes any stannousor stannic chlorides or sulphates remaining on the strip, but, due tothe fact that the major portion of the stannic or stannous chlorides 0rsulphates has been eliminated from the strip, the adverse effects of thehydrolyzing of the small remaining amount of stannous or stannic saltscan be eliminated by subjecting the strip to a suitable hot rinsingspray, such as is described and claimed in the copending applicationreferred to above, prior to the time that the strip passes to the flowbrightener.

From the above description, it will be apparent that the invention hasprovided a method and apparatus whereby the solution carried from theplating line by the strip can, in large measure, be recovered and thatthis can be accomplished without deleterious results to the finalproduct and Without deleteriously affecting the electroplating bath bylowering the pH value of the rinsing bath to the point Where very littleor no hydrolysis of the tin salts takes place during the recoveryoperation. The recovery of the solution carried from the plating line bythe strip effects a substantial saving in operating costs and, asstated, this can be accomplished Without adversely aifecting the finalproduct.

While the invention is described as applied to the manufacture of tinlate and while having referred only to electrolytic baths of thechloride or sulphate types, it will be understood that the invention maybe utilized with other electrolytic baths and in the manufacture ofother coated products.

What is claimed is:

l. The method of continuously and progressively manufacturingelectro-tinplated strip com prising the steps of passing strip throughan electrolytic cell containing an acid plating bath of aqueouselectrolyte solution including tin salt and fluorine ions, a source ofthe fluorine ions being a tin-free acidic fluorine compound present as acomponent of the electrolyte solution, electrodepositing a layer of tinon the strip while the strip is in contact With the electrolytesolution, thereafter passing the strip from the plating bath with tinsalt-containing electrolyte solution carried from the plating bath onthe strip, rinsing the strip with an aqueous rinse solution to removeelectrolyte solution including tin salts from the strip by alternatelyspraying the aqueous rinse solution onto the strip and squeezing theresulting solution from the strip, the first spraying and squeezingoperation diluting the concentration of tin salts in the solutionremaining on the strip and each subsequent spraying and squeezingoperation further diluting the concentration of tin salts in theresidual solution remaining on the strip, thereafter rinsing the stripwith an aqueous rinse to remove residual electrolyte solution includingtin salt remaining on the strip after being alternately sprayed andsqueezed, said last-mentioned aqueous rinse being obtained by admixingWater and a tin-free acidic fluorine compound, said tin-free acidicfluorine compound being in an amount suiiicient to reduce the pH valueof the aqueous rinse to a point Where hydrolysis of tin salt carriedfrom the plating bath on the strip and not removed by the alternatespraying and squeezing is substantially prevented, and utilizing aqueousrinse solution from said last-mentioned rinse step as the aqueous rinsesolution for said spraying steps.

The method of continuously and progressively manufacturingelectro-tinplated strip as forth in claim 1 wherein said tin-free acidicfluorine compound in the aqueous rinse is a sodium salt of hydrofluoricacid.

3. The method of continuously and progressively manufacturingelectro-tinplated strip as set forth in claim 1 wherein said tin-freeacidic fluorine compound in the aqueous rinse is sodium fluoride.

4. The method of continuously and progressi 1y manufacturingelectro-tinplated strip as set forth in ciaim 1 wherein said tin-freeacidic fluorine compound in the aqueous rinse is sodium bifluoride.

5. The method of continuously and progressively manufacturingelectro-tinplated strip as set forth in claim 1 wherein said tin-freeacidic fluorine compound in the aqueous rinse is hydroi'iuoric acid.

DORIS E. MORGAN, Formerly Doris E. Busc'r, administratrir of the estateof Joseph S. Buser, deceased.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,330,609 Nachtman Sept. 28, 1943 2,372,599 Nachtrnan Mar. 27,1945 2,407,579 Schweiker Sept. 10, 1946 FOREIGN PATENTS Number CountryDate 545,269 Great Britain July 3, 1942

1. THE METHOD OF CONTINUOUSLY AND PROGRESSIVELY MANUFACTURINGELECTRO-TINPLATED STRIP COMPRISING THE STEPS OF PASSING STRIP THROUGH ANELECTROLYTIC CELL CONTAINING AN ACID PLATING BATH OF AQUEOUS ELECTROLYTESOLUTION INCLUDING TIN SALT AND FLUORINE IONS, A SOURCE OF THE FLUORINEIONS BEING A TIN-FREE ACIDIC FLUORINE COMPOUND PRESENT AS A COMPONENT OFTHE ELECTROLYTE SOLUTION, ELECTRODEPOSITING A LAYER OF TIN ON THE STRIPWHILE THE STRIP IS IN CONTACT WITH THE ELECTROLYTE SOLUTION, THEREAFTERPASSING THE STRIP FROM THE PLATING BATH WITH TIN SALT-CONTAININGELECTROLYTE SOLUTION CARRIED FROM THE PLATING BATH ON THE STRIP, RINSINGTHE STRIP WITH AN AQUEOUS RINSE SOLUTION TO REMOVE ELECTROLYTE SOLUTIONINCLUDING TIN SALTS FROM THE STRIP BY ALTERNATELY SPRAYING THE AQUEOUSRINSE SOLUTION ONTO THE STRIP AND SQUEEZING THE RESULTING SOLUTION FROMTHE STRIP, THE FIRST SPRAYING AND SQUEEZING OPERATION DILUTING THECONCENTRATION OF TIN SALTS IN THE SOLUTION REMAINING ON THE STRIP ANDEACH SUBSEQUENT SPRAYING AND SQUEEZING OPERATION FURTHER DILUTING THECONCENTRATION OF TIN SALTS IN THE RESIDUAL SOLUTION REMAINING ON THESTRIP, THEREAFTER RINSING THE STRIP WITH AN AQUEOUS RINSE TO REMOVERESIDUAL